(1) Field of the Invention
The present invention relates to a high frequency (microwave) power amplifier for use in radio communication apparatuses in mobile radio communication systems, in multiplex radio communication systems, in satellite radio communication systems, and in broadcasting systems.
(2) Description of the Related Art
Conventionally, various microwave amplifiers are provided For example, a class "A" amplifier is suitable for a low distortion amplification, but a theoretical linear efficiency thereof is low (50%). This efficiency is improved to up to 78.5% in a class "B" amplifier, and the efficiency in a low power input in the class "B" amplifier is better than that of the class "A" amplifier, since the power consumption varies with an amount of power input. Nevertheless, the distortion is large when the power input is low. A class "C" amplifier can realize a 100% efficiency, but has an extremely poor distortion characteristic. Therefore, the class "C" amplifier is not suitable for an amplification of an electric signal wherein an envelope of a level of the electric signal varies, as in the case of a linear modulation system or a multi-carrier amplification system.
To constantly operate an amplifier, regardless of its power input level, with the same efficiency as that obtained when an amplification device in the amplifier is in a saturated condition, the amplification device must be always in the saturated condition, but this condition is contradictory to the linear amplification operation of the amplification device. In the prior art, the LINC system has been proposed as a way of solving this problem. The LINC system was proposed by D. C. Cox of the Bell laboratories ("Linear Amplification with Nonlinear Components", IEEE Transactions on Communications, COM-22, pp. 1942 to 1945, December 1974), and an application of the LINC system is proposed by Shigeru Tomisato et al. of the Nippon Telegraph and Telephone corporation ("Phase-Error Compensated LINC Modulator" reported in the autumn meeting of the Institute of Electronics, Information, and Communication Engineers in Japan, 1989). In the LINC system, a linear amplification of an electric signal with a high efficiency and without distortion can be obtained because a linear amplification can be realized using a non-linear amplifier, for example, the class "C" amplifier. The LINC system is explained below.
In the LINC system, an electric signal which has an envelope, and which is to be amplified, is decomposed to form two electric signals both having an equal and constant envelope, the decomposed signals are separately amplified by two respective amplifiers, and an amplified signal of the above electric signal is composed from the outputs of the amplifiers in a 90.degree. hybrid circuit.
Since, in the LINC system, the signals which are to be amplified each have a constant envelope, amplifiers with a high efficiency, for example, a class "C" amplifier, can be used for the amplification of the signals, and an original signal can be composed of the amplified signals. Namely, a linear amplification of an electric signal with an improved efficiency and without distortion is achieved because a linear amplification can be realized.
In the LINC system, however, the electric signals which are amplified in the above amplifiers have a constant envelope regardless of the power level of an input signal to the LINC system, and therefore, a constant power is consumed in each of the amplifiers regardless of the power level of an input signal. For example, the constant power is consumed in each of the amplifiers even when the power input is zero.
FIGS. 1A and 1B are diagrams for explaining a transformation of an input signal to form two electric signals each having a constant envelope, in the LINC system.
In the LINC system, a level of an envelope of an input signal is transformed to phases of the signals having a constant envelope, the transformed signals are separately amplified, and then the amplified signals are vector synthesized to generate an amplified signal of the input signal. As indicated in FIGS. 1A and 1B, an arbitrary input signal A having an amplitude of not more than two is decomposed to form two signals B and C, where the amplitudes of the signals B and C are made equal to one by equalizing the angle (phase) between the signal A and the transformed signal B, and the angle (phase) between the signal A and the transformed signal C. Generally, amplification devices operate most efficiently in the region near their saturated points, and the above transformed signals have a constant amplitude, the amplifiers (amplification devices) can be operated in the saturated region (non-linear region). The linearly amplified signal of the input signal A is obtained by vector synthesizing the amplified signals of the transformed signals B and C.
FIG. 2 is a diagram of a construction of the LINC system. In FIG. 2, reference numeral 21 denotes a constant envelope signal generation circuit which transforms an input signal to form two signals having a constant envelope and phases corresponding to an amplitude of the input signal; 22 and 23 each denote an amplifier which separately amplifies the transformed signals in their saturated regions; and 26 denotes a 90.degree. hybrid circuit. The outputs of the amplifiers 22 and 23 are input to two adjacent terminals of the 90.degree. hybrid circuit 26, and the above vector synthesized signal is obtained from one of the other terminals of the 90.degree. hybrid circuit 26. A dummy resistor 50 is connected to the last terminal of the 90.degree. hybrid circuit 26 and the earth. As shown in FIG. 2, in the LINC system, the power difference between a sum of the powers of the two transformed and amplified signals and the power of the synthesized signal, which difference corresponds to an unnecessary component, is transformed to heat in the dummy resistor 50.
A total efficiency .eta. of the LINC system is expressed as below. EQU .eta.t=Po.eta./2Pc
where Pc denotes the sum of powers of the transformed signals (constant envelope signals), .eta. is an efficiency of the amplifiers in the saturated condition, and Po denotes the power of the synthesized signal (the output of the LINC system). In the above system, the amplification devices are always saturated, and consume a constant power, and therefore, the total efficiency is proportional to the output power Pc of the system. Namely, the linearity of the amplification in the LINC system is improved, but the efficiency is not greatly improved. The efficiency is low when a difference between a peak power and an average power of an input signal is large because the power difference between a sum of the powers of the two transformed and amplified signals and the power of the output (synthesized) signal is consumed through the dummy resistor 50.
FIG. 3 is a diagram showing the efficiencies of an ideal class "A" amplifier, ideal class "B" amplifier, the LINC system containing amplifiers having an 100% efficiency, and constructions of various aspects of the present invention, which are explained later, for various values of relative output power. The efficiencies in the ideal class "A" amplifier and the LINC system are reduced in proportional to the output power, and the efficiency in the ideal class "B" amplifier is reduced in proportion to a square root of the output power.